1. Field of the Invention
The present invention relates to a carrier and a chemical mechanical polishing (CMP) apparatus for rotating and uniformly polishing a surface of a wafer or other workpiece while pressing it against a polishing pad of a platen.
2. Description of the Related Art
FIG. 41 is a sectional view of essential portions of a general CMP apparatus.
As shown in FIG. 41, this CMP apparatus is provided with a carrier 100 and a platen 110 on which a polishing pad 111 is attached. It is structured to polish the surface of a wafer W by the polishing pad 111 by making the carrier 100 and the platen 110 rotate by a not shown rotating mechanism while supplying a not shown polishing fluid in a state pressing the wafer W to the platen 110 side by a carrier base 101 of the carrier 100.
This CMP apparatus polishes the wafer W by the rear face reference polishing system, so a backing pad 102 is attached to the carrier base 101 and the surface of the wafer W is polished in a state with the backing pad 102 abutting against the back surface of the wafer W.
Meanwhile, uniformly polishing the surface of the wafer W requires that the sectional shapes of the backing pad 102 and the polishing pad 111 be uniform and that there be no variations in the thickness or the sectional shape.
Manufacturing a backing pad 102 or a polishing pad 111 with completely uniform sectional shapes, however, is difficult in practice. The backing pad 102 and the polishing pad 111 which are manufactured have considerable unevenness.
Accordingly, the distribution of pressure applied to the wafer W as a whole at the time of polishing is no longer uniform and the surface of the wafer W is not uniformly polished.
Further, even if the backing pad 102 and the polishing pad 111 are uniform in sectional shape, warping and waviness sometimes occur in the wafer W itself. As opposed to this, the CMP apparatus shown in FIG. 41 is not constructed to deal with such warping or waviness in the wafer W, so uneven polishing of the wafer W due to this warping and waviness occurs and it is not possible to uniformly polish the surface of the wafer W as a whole.
Further, when a disk shaped wafer W is pressed by the carrier 100, the pressure applied to the outer peripheral rim of the wafer W inevitably becomes larger than the pressure applied to other portions. This results in over polishing of the edges of the wafer W and a resultant poorer yield.
Therefore, as shown in FIG. 41, a retainer ring 103 is arranged at the outside of the wafer W and part of the pressure applied to the outer peripheral rim of the wafer W is relieved to the retainer ring 103 side so as to make the distribution of pressure applied to the wafer W as a whole uniform and try to improve the yield.
Whether or not the pressure at the outer peripheral rim of the wafer W becomes equal to the pressure at other portions, however, is determined by the amount of projection .DELTA. of the wafer W from the retainer ring 103.
Accordingly, it is necessary to adjust in advance the amount of projection of the retainer ring 103 from the carrier base 101 so that the amount of projection .DELTA. of the wafer W becomes the optimal value before polishing the wafer W.
When the polishing work is continued for a certain time, however, the retainer ring 103 in contact with the polishing pad 111 is worn down and the amount of projection .DELTA. starts to differ from the initial value. Therefore, it is necessary to stop the operation of the CMP apparatus each time and finely adjust the amount of projection of the retainer ring 103 so that the amount of projection .DELTA. becomes the optimal value. This has caused a decline in the operating rate.
As explained above, in a general CMP apparatus shown in FIG. 41, there were problems in the uniformity of polishing of the wafer W and the operating rate of the CMP apparatus due to the work for adjusting the retainer ring 103. Therefore, various types of CMP apparatuses have been considered to try to overcome these problems as much as possible using air pressure.
FIG. 42 is a sectional view of an air pressure type CMP apparatus according to a first example of the related art, FIG. 43 is a sectional view of an air pressure type CMP apparatus according to a second example of the related art; and FIG. 45 is a sectional view of an air pressure type CMP apparatus according to a third example of the related art.
The carrier 200 of the CMP apparatus according to the first example of the related art, as shown in FIG. 42, is constructed with a pressure chamber 202 provided below a carrier base 201.
Specifically, a ring-shaped narrow width abutment 203 is provided at a position inside the retainer ring 103 to define the pressure chamber 202. A silicone rubber sheet 204 at the lower surface of the abutment 203 abuts against the outer peripheral rim of the wafer W. In that state, air of a predetermined pressure is introduced into the pressure chamber 202.
Due to this, a uniform air pressure is applied to the wafer W to polish the wafer W by the front face reference polishing system.
Further, the carrier 300 of the CMP apparatus according to the second example of the related art, as shown in FIG. 43, is constructed with a ring-shaped projection 302 provided at an outer periphery of the lower surface of a carrier base 301, with a porous ceramic plate 303 attached to the inside of the projection 302, and with a perforated hard sheet 305 attached to the lower surface of the projection 302 by a two-sided adhesive tape 301 so as to define a pressure chamber 306. Further, a perforated backing pad 307 is adhered to the lower surface of the perforated sheet 305 and the retainer ring 103 is adhered to a portion of the lower surface of the perforated sheet 305 corresponding to the projection 302 by a two-sided adhesive tape 308.
By this construction, the air passing through the air holes in the carrier base 301 is introduced through the perforated ceramic plate 303 to the pressure chamber 306. The air pressure in the pressure chamber 306 uniformly presses against the wafer W through the perforated sheet 305 and the backing pad 307.
Further, the CMP apparatus according to the third example of the related art is the art disclosed in Japanese Patent Laid-Open No. 7-171757. A carrier 400, as shown in FIG. 45, is constructed with a flexible thin sheet 402 attached to the bottom part of a wafer holding member 401 and with the wafer holding member 401 suspended from a housing 405 through an expandable cylindrical member 403 and a high flexibility support member 404. Due to this construction, the inside of the wafer support member 401 is pressurized at positive pressure and a uniform air pressure is applied to the wafer W in the state with the wafer W sucked against the flexible thin sheet 402 by a flexible hose 406.
The CMP apparatuses of the above related art, however, suffered from the following problems:
In the carrier 200 of the CMP apparatus according to the first example of the related art shown in FIG. 42, a uniform air pressure was applied to the upper surface of the wafer W to polish the wafer W by the front face reference polishing system, so there was almost no effect received due to the sectional shape of the polishing pad 111 or warping etc. of the wafer W and the surface of the wafer W could be polished uniformly.
This CMP apparatus, however, sets the lateral width of the abutment 203 small so as to secure a pressurization area for the air. Accordingly, the area of contact between the outer peripheral rim of the wafer W and the silicone rubber sheet 204 becomes smaller, air in the pressure chamber 202 leaks from below the silicone rubber sheet 204, the pressure applied to the outer peripheral rim of the wafer W becomes uneven, and this portion is not uniformly polished.
Further, in this CMP apparatus, the margin of the amount of projection .DELTA. of the wafer W is too small.
That is, the amount of projection of the retainer ring 103 from the carrier base 201 is determined so that the amount of projection .DELTA. of the wafer W becomes the optimal value, but the retainer ring 103 is worn down along with the polishing work. When the retainer ring 103 is worn down by as much as 1 .mu.m from the optimal value, it is not enough to immediately adjust the amount of projection of the retainer ring 103.
That is, there is an allowable range (margin) to the amount of wear of the retainer ring 103. So long as the amount of wear is within this allowable range, the amount of over polishing of the outer peripheral rim of the wafer W is small and it is possible to prevent a reduction in the yield.
In the carrier 200 of this CMP apparatus, however, the outer peripheral rim of the wafer W is pushed in by the narrow width abutment 203, so the margin of the amount of wear of the retainer ring 103 is a small 10 .mu.m and it is necessary to adjust the amount of projection of the retainer ring 103 in a short time. Proposals for improving on this have been awaited.
Further, in the carrier 300 of the CMP apparatus according to the second example of the related art shown in FIG. 43, it is possible to apply pressure to the entire surface of the wafer W by a uniform air pressure, but the range of over polishing at the outer peripheral rim of the wafer W is large and the yield becomes remarkably poor.
FIG. 44 is an enlarged sectional view of the state of over polishing. As shown in FIG. 44, the carrier 300 is constructed with the projection 302 of the carrier base 301 and the retainer ring 103 gripping the outer periphery of the perforated sheet 305 covering the wafer W, so the outer periphery of the perforated sheet 305 is pulled downward during the polishing work.
Accordingly, a tension T occurs at the outer periphery of the perforated sheet 305 and a pressure of a vertical component T1 of the tension T is applied to the outer peripheral rim of the wafer W in addition to the air pressure P.
As a result, the polishing rate of the outer peripheral rim of the wafer W becomes remarkably larger than the polishing rate of other portions, the range of over polishing L becomes as high as 10 mm to 20 mm, and the yield of the wafer W ends up becoming remarkably poor.
Further, while probably due to this, the margin of the amount of wear of the retainer ring 103 also is a small 20 .mu.m or so.
In addition, in the carrier 400 of the CMP apparatus according to the third example of the related art shown in FIG. 45, since it is not possible to apply a positive pressure in the wafer holding member 401 to the center of the flexible thin sheet 402 where the flexible hose 406 is attached, application of completely uniform air pressure is not possible. Further, since a center hole 402a of the flexible thin sheet 402 is negative in pressure, uneven polishing occurs at the center of the wafer W.
Further, since the wafer holding member 401 is constructed to be suspended from the housing 405, the carrier 400 ends up becoming large in size. Further, it is necessary to balance the air pressure inside the wafer holding member 401 and the weight of the wafer holding member 401 to eliminate any effect of the weight of the wafer holding member 401, so it is not possible to freely adjust the air pressure in the wafer holding member 401. As a result, slight fluctuations in the air inside the wafer holding member 401 end up having a large effect on the uniformity of polishing of the wafer W.
Further, the carrier 400, like the carrier 200 shown in FIG. 43, has the outer periphery of the flexible thin sheet 402 pulled downward, so the tension of the flexible thin sheet 402 causes the polishing rate of the outer peripheral rim of the wafer W to become remarkably larger than the polishing rate of the other portions.